my grandsons are 1 1/2 and 3 1/2 years old. they have been to disneyworld 4 times. in retrospect, we can tell you that the trip when the first was only 3 months old was the easiest by far! as my daughter (their mother) puts it, "he's still just a sack of potatoes we have to carry around, an adorable sack, but a sack". really, other than seeing to his basic needs, he is not that big a change from your prior trips. usually at 3 months old, they'll nap anywhere, eat only one food, mess a couple diapers. you will worry a lot, but he will be fine just being snuggled up against you or napping in a stroller.
once they are about 6 months old or more, you start centering the trip on what you can do to make them happy/ enjoy the trip. you want to expose them to the sights, the sounds, the parades, etc.
go now, it's the last adult type trip you will take, believe me, after this, it's all about the babe's enjoyment.
also- consider garden grocer. at least if the baby is formula fed not nursed. we always have diapers, wipes and formula delivered to the hotel. a lot easier than using valuable suitcase space for them. not sure it is worth it if you don't also need formula, just diapers and wipes. but keep it in mind as the babe grows. we usually get diapers, wipes, formula, disposable bibs, baby food and then throw in some adult/room snacks, breakfasts as long as they are delivering.
PS- C0NGRATULATIONS!!!